(Blender 2.8 Stable) Smoke Domain renders as cube in EEVEE
You have a Principled BSDF shader connected to the surface of the domain object. Objects with volumetric materials generally should not have any shader connected to the surface slot. Simply remove the principled BSDF shader, and leave the principled volume shader, and it should work. I 

You want your node setup to look like this: 


Hope this helps!